Process for recovering a crystalline product from solution

ABSTRACT

An improved process for recovering a crystalline product (particularly an N-(phosphonomethyl)glycine product) from a solution comprising both a product subject to crystallization and undesired impurities is provided.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/943,783, now U.S. Pat. No. 7,179,936, filed Sep. 17, 2004, whichclaims the benefit of U.S. Provisional Application Ser. No. 60/504,466,filed Sep. 17, 2003, and U.S. Provisional Application Ser. No.60/558,518, filed Mar. 31, 2004, the entire contents of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to processes for producing andrecovering a crystalline product from a solution comprising a productsubject to crystallization and undesired impurities. More particularly,the invention relates to processes for producing and recoveringN-(phosphonomethyl)glycine products from aqueous reaction solutionsprepared by the liquid phase oxidation ofN-(phosphonomethyl)iminodiacetic acid substrates.

BACKGROUND OF THE INVENTION

N-(phosphonomethyl)glycine is described by Franz in U.S. Pat. No.3,799,758. N-(phosphonomethyl)glycine and its salts are convenientlyapplied as a component of aqueous, post-emergent herbicide formulations.As such, they are particularly useful as a highly effective andcommercially important broad-spectrum herbicide for killing orcontrolling the growth of a wide variety of plants, includinggerminating seeds, emerging seedlings, maturing and established woodyand herbaceous vegetation and aquatic plants.

One of the more widely accepted methods of makingN-(phosphonomethyl)glycine products includes the liquid phase oxidativecleavage of a carboxymethyl substituent from anN-(phosphonomethyl)iminodiacetic acid substrate. As used herein,“N-(phosphonomethyl)iminodiacetic acid substrates” includeN-(phosphonomethyl)iminodiacetic acid and salts thereof, wherein thesalt-forming cation is, for example, ammonium, alkylammonium, an alkalimetal or an alkaline earth metal. Over the years, a wide variety ofmethods and reactor systems have been disclosed for conducting thisoxidation reaction. See generally, Franz, et al., Glyphosate: A UniqueGlobal Herbicide (ACS Monograph 189, 1997) at pp. 233-62 (and referencescited therein); Franz, U.S. Pat. No. 3,950,402; Hershman, U.S. Pat. No.3,969,398; Felthouse, U.S. Pat. No. 4,582,650; Chou, U.S. Pat. No.4,624,937; Chou, U.S. Pat. No. 4,696,772; Ramon et al., U.S. Pat. No.5,179,228; Siebenhaar et al., International Publication No. WO 00/01707;Ebner et al., U.S. Pat. No. 6,417,133; Leiber et al., U.S. Pat. No.6,586,621; and Haupfear et al., International Publication No. WO01/92272.

The liquid phase oxidation of an N-(phosphonomethyl)iminodiacetic acidsubstrate typically produces a reaction mixture containing water andvarious impurities besides the desired N-(phosphonomethyl)glycineproduct. These impurities may include, for example, various by-products,unreacted starting materials, as well as impurities present in thestarting materials. Representative examples of impurities present inN-(phosphonomethyl)glycine product reaction mixtures include unreactedN-(phosphonomethyl)iminodiacetic acid substrate,N-formyl-N-(phosphonomethyl)glycine, phosphoric acid, phosphorous acid,hexamethylenetetraamine, aminomethylphosphonic acid (AMPA), methylaminomethylphosphonic acid (MAMPA), iminodiacetic acid (IDA),formaldehyde, formic acid, chlorides and the like. The value of theN-(phosphonomethyl)glycine product normally dictates maximal recovery ofthe product from the reaction mixture and also often provides incentivefor recycling at least a portion of the depleted reaction mixture (e.g.,to the oxidation reactor system) for further conversion of unreactedsubstrate and recovery of product.

Commercial considerations also sometimes dictate that the concentrationof the N-(phosphonomethyl)glycine product in the commercially soldmixtures be significantly greater than the concentrations in thereaction mixtures that are typically formed in the oxidation reactorsystem, particularly where the N-(phosphonomethyl)glycine product isbeing stored or shipped for agricultural applications. For example, whena heterogeneous catalyst is used for the liquid phase oxidation ofN-(phosphonomethyl)iminodiacetic acid to make theN-(phosphonomethyl)glycine as described by Haupfear et al. inInternational Publication No. WO 01/92272, it is typically preferred tomaintain a maximum concentration of the N-(phosphonomethyl)glycineproduct in the reaction mixture of no greater than about 9% by weight inorder to keep the product solubilized, although higher concentrations inexcess of 9% and even up to about 12% by weight may be suitably utilizedat higher reaction mixture temperatures. Sometimes, however, it isdesirable for the commercially sold mixtures to have anN-(phosphonomethyl)glycine concentration that is significantly greater.Thus, after the N-(phosphonomethyl)glycine product has been formed and,if necessary, separated from the catalyst, it is typically preferable toconcentrate the product and separate the product from the variousimpurities in the oxidation reaction mixture.

Smith, in U.S. Pat. No. 5,087,740, describes one process for purifyingand concentrating an N-(phosphonomethyl)glycine product. Smith disclosespassing a reaction mixture containing N-(phosphonomethyl)glycine througha first ion exchange resin column to remove impurities that are moreacidic than the N-(phosphonomethyl)glycine, passing the effluent fromthe first ion exchange resin column through a second ion exchange resincolumn which adsorbs the N-(phosphonomethyl)glycine, and recovering theN-(phosphonomethyl)glycine by passing a base or strong mineral acidthrough the second ion exchange resin column.

Haupfear et al., in International Publication No. WO01/92272, describeprocesses for purifying and concentrating an N-(phosphonomethyl)glycineproduct prepared by the oxidation of N-(phosphonomethyl)iminodiaceticacid substrates. Haupfear et al. describe generating two crystallineN-(phosphonomethyl)glycine products in two separate crystallizerswherein the crystals have two distinct purities. The lower puritymaterial may then be blended with the higher purity material to producea single product of acceptable purity.

There remains a need for processes for producing and recovering acrystalline product from a solution comprising a product subject tocrystallization and undesired impurities that is capable of producingmultiple product mixtures containing the crystalline product, eachexhibiting a suitable impurity profile for the intended use.Particularly a need exists for processes for producing and recovering acrystalline N-(phosphonomethyl)glycine product from a reaction solutionprepared by the oxidation of an N-(phosphonomethyl)iminodiacetic acidsubstrate capable of producing both a saleableN-(phosphonomethyl)glycine wet-cake product as well as concentratedliquid or solid salts of N-(phosphonomethyl)glycine of acceptable purityfor use in formulation of herbicidal compositions. Such a process wouldimprove overall flexibility to adequately support market demand forvarious N-(phosphonomethyl)glycine products.

SUMMARY OF THE INVENTION

Among certain objects of the present invention, therefore, are theprovision of an improved process for the recovery of one or morecrystalline products from a solution comprising both a product subjectto crystallization from the solution and undesired impurities; theprovision of such a process wherein one or more crystalline products ofacceptable purity may be produced without washing the crystallineproduct; the provision of such a process which is capable of recoveringone or more crystalline products of acceptable purity when cake washingis insufficient to remove occluded impurities from the crystallineproduct; the provision of a process for recovering one or moreN-(phosphonomethyl)glycine products from a slurry comprisingprecipitated N-(phosphonomethyl)glycine product crystals and a motherliquor; the provision of such a process capable of producing a pluralityof suitable crystalline products, for example, multiple wet-cakeproducts, thereby providing for greater process flexibility; and theprovision of such a process that can recover crystalline products ofacceptable purity having better handling and packaging characteristics.

Briefly therefore, one aspect of the present invention is directed toprocesses for recovering an N-(phosphonomethyl)glycine product from aslurry comprising precipitated N-(phosphonomethyl)glycine productcrystals and a mother liquor. In a first embodiment, the processcomprises dividing the slurry into plural fractions comprising a firstslurry fraction and a second slurry fraction. PrecipitatedN-(phosphonomethyl)glycine product crystals are separated from the firstand second slurry fractions to produce a first wet-cake product and asecond wet-cake product, respectively. The ratio of solids content ofthe second wet-cake product to the solids content of the first wet-cakeproduct, as measured by weight percent of solids in the first and secondwet-cake products, is at least about 1.1.

In another embodiment, the process for recovering anN-(phosphonomethyl)glycine product from a slurry comprising precipitatedN-(phosphonomethyl)glycine product crystals and a mother liquorcomprises dividing the slurry into plural fractions comprising first andsecond slurry fractions. The first slurry fraction is introduced into afirst liquid/solids separation device in which precipitatedN-(phosphonomethyl)glycine product crystals are separated from the firstslurry fraction to produce a first wet-cake product. The second slurryfraction is introduced into a second liquid/solids separation device inparallel with the first liquid/solids separation device and in whichprecipitated N-(phosphonomethyl)glycine product crystals are separatedfrom the second slurry fraction to produce a second wet-cake product.The second wet-cake product has a greater solids content than the firstwet-cake product as measured by weight percent of solids in the wet-cakeproducts.

Other objects and features of this invention will be in part apparentand in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow sheet of one embodiment of the presentinvention employing two separate liquid/solids separation systems ordevices in parallel for the recovery of wet-cake products havingdifferent solids contents from a product slurry produced byconcentrating a solution containing a product subject to crystallizationtherefrom in a crystallization stage.

FIG. 2 is a schematic flow sheet of an integrated process for oxidizingan N-(phosphonomethyl)iminodiacetic acid substrate in a reactor systemto form an oxidation reaction solution comprising anN-(phosphonomethyl)glycine product and for recovering two crystallineN-(phosphonomethyl)glycine wet-cake products from the oxidation reactionsolution using a first adiabatic and a second non-adiabatic evaporativecrystallization stages and separate liquid/solids separation systems ordevices in parallel.

FIG. 3 is a schematic flow sheet of a process as described in FIG. 2wherein impurity distribution among the crystallineN-(phosphonomethyl)glycine wet-cake products is managed by net transferof product slurry or magma from a first adiabatic crystallization trainto the product slurry of a second non-adiabatic evaporativecrystallization train.

FIG. 4 is a schematic flow sheet of a process as described in FIG. 2wherein impurity distribution among the crystallineN-(phosphonomethyl)glycine wet-cake products is managed by net transferof product slurry from a first adiabatic crystallization train to asecond non-adiabatic evaporative crystallization stage.

FIG. 5 is a schematic flow sheet of a process as described in FIG. 2wherein impurity distribution among the crystallineN-(phosphonomethyl)glycine wet-cake products is managed by net transferof N-(phosphonomethyl)glycine product crystals from a first adiabaticcrystallization train to the product slurry of a second non-adiabaticevaporative crystallization train.

FIG. 6 is a schematic flow sheet of a process as described in FIG. 2wherein impurity distribution among the crystallineN-(phosphonomethyl)glycine wet-cake products is managed by net transferof impurities contained in mother liquor from a first adiabaticcrystallization operation and/or a second non-adiabatic evaporativecrystallization operation to: (i) the other of the adiabatic and/orevaporative crystallization operations; (ii) the other of the adiabaticand/or evaporative liquid/solids separation steps; (iii) the other ofthe adiabatic or evaporative wet-cake products; or any combination of(i), (ii) and/or (iii).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, improvements in processes forproducing and recovering multiple crystalline wet-cake products(particularly N-(phosphonomethyl)glycine wet-cake products) from one ormore solutions comprising both a product subject to crystallization andundesired impurities have been discovered. Typically, at least one ofthe crystalline products recovered is of acceptable purity, and anyother crystalline products recovered are of acceptable purity, can beblended with one or more other crystalline products to form a product ofacceptable purity, and/or can be further processed or blended to form awet-cake or concentrated liquid or solid salts ofN-(phosphonomethyl)glycine of acceptable purity for use in formulationof herbicidal compositions. Typically, N-(phosphonomethyl)glycinewet-cake of acceptable purity contains at least about 95% by weightN-(phosphonomethyl)glycine product (dry basis) and the remainder isimpurities such as reaction by-products, unreacted starting materialsand impurities present in the starting materials. Individual impuritiesmay have individual concentration specifications.

Without being held to a particular theory, it has been discovered thatby separating a plurality of wet-cake products having different solidscontents, different impurity concentrations, and/or different crystalsize distributions from one or more product slurries comprising aprecipitated product and impurities, the impurity content of thewet-cake products can be more effectively managed, thereby providingincreased process flexibility. The process of the present invention isparticularly advantageous for the concentration and recovery ofcrystalline products in processes wherein a conventional cake-washingstep is either undesirable or insufficient to produce products ofacceptable purity. For example, the process of the present invention hasbeen discovered to be effective for producing wet-cake products ofacceptable purity even when the product crystals precipitated fromsolution contain occluded impurities or impurities incorporated in thesolids by other means which cannot be removed efficiently or practicallyby conventional cake washing or by other measures such as reslurryingwith water or recrystallization. Further, the improved process of thepresent invention may also allow for the preparation of wet-cakeproducts exhibiting improved packaging and handling characteristics.

It is important to note that the strategies set forth herein have wideapplication in processes for preparing reaction solutions comprisingproducts subject to crystallization and concentrating and recoveringcrystallized product wet-cakes from the reaction solutions. The presentinvention has particular application in the concentration and recoveryof wet-cake products from oxidation reaction solutions containingN-(phosphonomethyl)glycine product susceptible to crystallization andespecially those containing N-(phosphonomethyl)glycine, wherein theoxidation reaction solution is produced by the catalytic liquid phaseoxidation of an N-(phosphonomethyl)iminodiacetic acid substrate.However, it should be understood that the present invention is equallyapplicable to recovering wet-cake products from solutions containingN-(phosphonomethyl)glycine product produced by routes other thancatalytic liquid phase oxidation of an N-(phosphonomethyl)iminodiaceticacid substrate that are well known to those skilled in the art.

As is recognized in the art, the liquid phase oxidation ofN-(phosphonomethyl)iminodiacetic acid substrates may be carried out in abatch, semi-batch or continuous reactor system containing one or moreoxidation reaction zones. The oxidation reaction zone(s) may be suitablyprovided by various reactor configurations, including those that haveback-mixed characteristics, in the liquid phase and optionally in thegas phase as well, and those that have plug flow characteristics.Suitable reactor configurations having back-mixed characteristicsinclude, for example, stirred tank reactors, ejector nozzle loopreactors (also known as venturi-loop reactors) and fluidized bedreactors. Suitable reactor configurations having plug flowcharacteristics include those having a packed or fixed catalyst bed(e.g., trickle bed reactors and packed bubble column reactors) andbubble slurry column reactors. Fluidized bed reactors may also beoperated in a manner exhibiting plug flow characteristics. Theconfiguration of the oxidation reactor system, including the number ofoxidation reaction zones and the oxidation reaction conditions are notcritical to the practice of the present invention. Suitable oxidationreactor systems and oxidation reaction conditions for liquid phasecatalytic oxidation of an N-(phosphonomethyl)iminodiacetic acidsubstrate are well-known in the art and described, for example, by Ebneret al., U.S. Pat. No. 6,417,133, by Leiber et al., U.S. Pat. No.6,586,621, and by Haupfear et al., International Publication No. WO01/92272 and corresponding U.S. Publication No. US-2002-0068836-A1, theentire disclosures of which are incorporated herein by reference.

The process described herein has been found to be particularly useful inrecovering multiple N-(phosphonomethyl)glycine wet-cake products fromoxidation reaction solutions produced by various continuous oxidationreactor systems described, for example, by Haupfear et al. inInternational Publication No. WO 01/92272. However, it is important tonote that the present invention is not limited to such applications orto use in conjunction with continuous oxidation reactor systemsgenerally. As will be apparent to those skilled in the art, thestrategies set forth herein may be advantageously applied in recoveringcrystalline wet-cake products from oxidation reaction solutions producedin a wide variety of reactor systems, including batch reactor systems.

Generally, in one embodiment, the process of the present inventioncomprises recovering separate wet-cake products from a slurry comprisingprecipitated product crystals and a mother liquor. The product slurry isdivided into plural fractions comprising at least a first fraction and asecond fraction. Product crystals are separated from each of the firstand second fractions by dewatering in one or more liquid/solidsseparation devices to produce a first wet-cake product and a secondwet-cake product, respectively.

More particularly, it has been found that the impurity content of aseparated wet-cake product may be maintained below a desired value bygenerating at least two wet-cake products from a slurry comprisingprecipitated product crystals and a mother liquor such that the solidscontent of the second wet-cake product is greater than the solidscontent of the first wet-cake product. Thus, it is necessary inpracticing this aspect of the present invention for the first and secondwet-cake products to have different solids contents resulting in eachwet-cake product having a different impurity composition due to thedifferent amounts of impurity-containing mother liquor in the wet-cake.For example, the ratio of the solids content of the second wet-cakeproduct to the solids content of the first wet-cake product, as measuredby weight percent of solids in each of the first and second wet-cakeproducts, is typically at least about 1.1. Preferably, the ratio of thesolids content of the second wet-cake product to the solids content ofthe first wet-cake product, as measured by weight percent of solids ineach of the first and second wet-cake products, is at least about 1.2.More preferably, the ratio of the solids content of the second wet-cakeproduct to the solids content of the first wet-cake product, as measuredby weight percent of solids in each of the first and second wet-cakeproducts, is at least about 1.25.

In accordance with a preferred embodiment, the solids content of thesecond wet-cake product is preferably at least about 85% by weightsolids. More preferably, the second wet-cake product has a solidscontent of from about 90% by weight solids to about 99% by weightsolids. Most preferably, the second wet-cake product has a solidscontent of from about 95% by weight solids to about 99% by weightsolids. Generally, increasing the solids content of the second wet-cakeproduct allows the recovery of a greater quantity of the second wet-cakeproduct of acceptable purity. Likewise, it is preferred that the firstwet-cake product has a solids content of less than about 85% by weightsolids. More preferably, the first wet-cake product has a solids contentof less than about 75% by weight solids. For example, the first wet-cakeproduct may have a solids content of from about 70% by weight solids toabout 85% by weight solids. It should be understood that as impuritylevels in the crystallization feed solution are reduced, product crystalsize tends to increase, resulting in more efficient dewatering andhigher solids content in the wet-cake products.

While not necessary or critical to the invention, it is contemplatedthat the first and second wet-cake products may typically be producedusing separate liquid/solids separation devices, preferably separateliquid/solids separation devices arranged or operated in parallel.Generally, any liquid/solids separation device suitable for separating acrystal product from a mother liquor may be used in the presentinvention. However, because of the relatively high throughput andcapacity requirements required by processes for the concentration andrecovery of N-(phosphonomethyl)glycine products from a reaction solutionresulting from the liquid phase oxidation ofN-(phosphonomethyl)iminodiacetic acid substrates, preferred embodimentsof the present invention typically employ liquid/solids separationdevices adapted for pressure filtration, vacuum filtration, and/orcentrifugation. For example, preferred liquid/solids separation devicesmay include vacuum drums, vacuum table filters and/or centrifuges. In aparticularly preferred embodiment, product crystals are separated fromthe first and second slurry fractions by centrifugation, preferably inseparate centrifuges, and even more preferably in separate centrifugesoperated in parallel. In an especially preferred embodiment, the firstwet-cake product is separated in a solid bowl centrifuge and the secondwet-cake product is separated in a basket centrifuge (or a bank ofbasket centrifuges). Alternatively, it is contemplated that productcrystals may be separated from the first and second slurry fractions insimilar liquid/solids separation devices and/or under conditions suchthat the wet-cakes as initially produced have relatively equal solidscontents. In such an embodiment, it may be possible to obtain therequired solids content ratio within the first and second wet-cakeproducts by combining the wet-cake product with mother liquor separatedfrom either of the first or second product slurry fractions (i.e., bysending forward separated mother liquor for combination with thewet-cake product either directly or in a subsequent processing step).

A particularly preferred embodiment wherein product crystals areseparated from first and second fractions of the product slurry inseparate liquid/solids separation devices operated in parallel isillustrated in FIG. 1. A feed solution 1 comprising a product subject tocrystallization is introduced into a crystallization stage 3 to producea crystalline product slurry or magma 5 comprising precipitated productcrystals and a mother liquor. For example, a product slurry comprisingN-(phosphonomethyl)glycine product crystals and mother liquor may beproduced by steam-driven evaporative crystallization, adiabaticcrystallization or adiabatic crystallization with decantation of areaction solution resulting from the catalytic liquid phase oxidation ofan N-(phosphonomethyl)iminodiacetic acid substrate. An overhead vaporstream 7 is removed from the crystallization stage.

The product slurry 5 is divided into a first fraction 9 and a secondfraction 11. The proportion of the product slurry divided into the firstand second fractions may vary considerably. For example, first fraction9 divided from slurry 5 may constitute from about 20% to about 100%,from about 40% to about 60%, or about 50% of the slurry and the secondfraction 11 constitute the remainder of the slurry.

The first slurry fraction 9 is introduced into a first liquid/solidsseparation device 13 such as a centrifuge, preferably a solid bowlcentrifuge, to produce a first wet-cake product 15 and a solids-depletedstream 17 (e.g., centrate) that is typically recycled back to thecrystallization stage 3. However, at least a portion of thesolids-depleted stream 17 can optionally be mixed back in with thewet-cake 15 as shown by the dashed line in FIG. 1, to generate a firstwet-cake product of even lower solids content. Furthermore, at least aportion of the solids-depleted stream 17 can optionally be mixed back inwith the wet-cake 15 in a later processing step.

The second product slurry fraction 11 can optionally be introduced intoa hydroclone (or bank of hydroclones) 19 to form a concentrated secondslurry fraction 23 enriched in precipitated product and asolids-depleted stream 21. The concentrated second fraction 23 isintroduced into a separator feed tank 25 that feeds a secondliquid/solids separation device, preferably a basket centrifuge.Alternatively, product slurry fraction 11 can be fed directly into theseparator feed tank 25 or directly into the second liquid/solidsseparation device. In the preferred embodiment shown in FIG. 1, theconcentrated second fraction is introduced into a bank of basketcentrifuges. Thus, the concentrated second fraction 23 accumulated inthe separator feed tank 25 is divided into concentrated slurry fractions27A and 27B that are introduced into basket centrifuges 29A and 29B,respectively. The basket centrifuges produce a wet-cake product 31A and31B, respectively, and these are combined to form a second wet-cakeproduct 35. The basket centrifuges further produce centrates 33A and 33Bthat are further depleted in precipitated product and may be recycledback to the crystallization stage 3. However, at least a portion ofcentrates 33A and/or 33B can optionally be mixed back in with wet-cakeproducts 31A, 31B and/or second wet-cake product 35 or mixed with firstwet-cake product 15 to generate wet-cake products of even lower solidscontent.

As noted above, the liquid/solids separation devices used to dewaterfirst and second product slurry fractions 9 and 11 in FIG. 1 arepreferably a solid bowl centrifuge and one or more basket centrifuges,respectively. Where the first wet-cake product can contain more waterand impurities without compromising the product specification, the useof a solid bowl centrifuge in conjunction with vertical basketcentrifuges provides a higher solid capacity capability while requiringlower capital and operating costs.

In the embodiment shown in FIG. 1, the second wet-cake product 35 wouldhave a lower impurity level than the first wet-cake product 15 due tothe lower amount of entrained mother liquor in the wet-cake product.Furthermore, the second wet-cake product 35 typically has an impuritylevel below the required specification and an N-(phosphonomethyl)glycineproduct assay of at least about 95% by weight on a dry basis such thatit can be packaged as a final product or used as feedstock in asubsequent processing step, for example, in the preparation ofconcentrated liquid or solid salts of N-(phosphonomethyl)glycine for usein formulation of herbicidal compositions. The first wet-cake product 15as obtained may or may not meet the applicable purity specification, butcan be utilized in conjunction with further processing (e.g., mixingwith higher purity N-(phosphonomethyl)glycine product orrecrystallization) to also produce a material or product of acceptablepurity having different properties than the second wet-cake product.

The embodiment shown in FIG. 1 can be part of a process where stage 3 isthe only crystallization step in the process. However, the embodimentshown in FIG. 1 can also be part of a broader process that containsother crystallization stages, as described below in connection with FIG.2.

In a particularly preferred embodiment, the present invention involvesproducing and recovering multiple wet-cakes containing crystallineN-(phosphonomethyl)glycine product from an oxidation reaction solutioncomprising N-(phosphonomethyl)glycine product and impurities in aprocess utilizing at least two crystallization stages operating in asemi-parallel manner.

Referring now to FIG. 2, an aqueous feed stream 101 comprising anN-(phosphonomethyl)iminodiacetic acid substrate is introduced along withoxygen into an oxidation reactor system 103 comprising one or moreoxidation reaction zone(s), wherein the N-(phosphonomethyl)iminodiaceticacid substrate is oxidatively cleaved in the presence of a suitablecatalyst to form an aqueous oxidation reaction solution 105 comprisingN-(phosphonomethyl)glycine product and impurities. In order to reducethe impurity level in the oxidation reaction solution 105, the catalystemployed in the oxidation reaction zone(s) is preferably a heterogeneouscatalyst comprising a noble metal on a carbon support, for example, asdescribed by Ebner et al., U.S. Pat. No. 6,417,133. The oxidationreaction solution 105 withdrawn from reactor system 103 is then dividedinto plural fractions and a portion 107 (i.e., a primary fraction of theoxidation reaction solution) is introduced and concentrated in a firstcrystallizer 111 that operates substantially adiabatically (i.e., anyheat input or removal to the crystallizer is no greater than about 200kcal/kg of oxidation reaction solution fed to the crystallizer) toproduce a primary product slurry or magma 113 comprising precipitatedN-(phosphonomethyl)glycine product crystals and primary mother liquor.Another portion 109 (i.e., a secondary fraction of the oxidationreaction solution) is introduced and concentrated in a non-adiabaticheat-driven evaporative crystallizer 125 to produce an evaporativecrystallization slurry or magma 126 (i.e., a secondary product slurry)comprising precipitated N-(phosphonomethyl)glycine product crystals andsecondary mother liquor.

Suitable operation of adiabatic crystallizer 111 and non-adiabaticcrystallizer 125 in the product recovery system shown in FIG. 2 isgenerally described by Haupfear et al. in International Publication No.WO 01/92272 and corresponding U.S. Publication No. US-2002-0068836-A1,which are incorporated herein by reference. As described in thispublication, adiabatic crystallizer 111 provides three differentfunctions, including: flash vaporization of a fraction of the oxidationreaction solution, crystallization of N-(phosphonomethyl) glycineproduct by the cooling induced by the vacuum operation of thecrystallizer, and subsequent decantation of a large portion of thecrystallization mother liquor for recycle to the reactor system. Thisdecantation also serves to concentrate the solids content of the primaryproduct slurry fed to the liquid/solids separation device for reduceddewatering load and increased dewatering capacity. These functions canbe provided integrally in single adiabatic crystallizer apparatus, or ina combination of apparatus.

Preferably, from about 30% to about 85%, more preferably from about 50%to about 80%, and even more preferably from about 65% to about 75% ofthe oxidation reaction solution 105 is introduced into the adiabaticcrystallizer 111 via stream 107 as the primary fraction, while theremaining portion is introduced into the non-adiabatic heat-drivencrystallizer 125 via stream 109 as the secondary fraction. The weightratio of the secondary fraction 109 to theN-(phosphonomethyl)iminodiacetic acid substrate fed into the reactorsystem 103 is preferably from about 0.1 to about 9, more preferably fromabout 0.2 to about 5, even more preferably from about 0.25 to about 2.5.However, the proportion of the oxidation reaction solution 105introduced into the adiabatic crystallizer 111 and the weight ratio ofthe secondary fraction 109 to the N-(phosphonomethyl)iminodiacetic acidsubstrate fed into the reactor system 103 are not narrowly critical inthe practice of the present invention.

Operation of the adiabatic crystallizer 111 produces vapor 115 (i.e.,the adiabatic crystallizer overhead) discharged from the top of thecrystallizer, a decantate (i.e., primary mother liquor) stream 112withdrawn from the crystallizer and the primary crystallization productslurry 113 removed from the bottom of the crystallizer and comprisingprecipitated crystalline N-(phosphonomethyl)glycine product and primarymother liquor containing uncrystallized (i.e., dissolved)N-(phosphonomethyl)glycine product and impurities. Preferably, at leasta portion (and more preferably all) of the adiabatic crystallizeroverhead 115 and/or decantate 112 withdrawn from the adiabaticcrystallizer 111 is/are recycled back to the oxidation reactor system103.

The primary crystallization product slurry 113 comprising precipitatedcrystalline N-(phosphonomethyl)glycine product and primary mother liquorremoved from the bottom of the adiabatic crystallizer 111 is introducedinto a liquid/solids separation device 117, preferably a basketcentrifuge or bank of basket centrifuges, to produce a wet-cake product119 and a solids-depleted stream 123 (e.g., centrate). At least aportion of the solids-depleted stream 123 may be recycled back to theadiabatic crystallizer 111 and/or optionally may be recycled back to theoxidation reactor system 103 as shown by the dashed line in FIG. 2.Preferably, the wet-cake product 119 has a solids content of from about90% to about 99% by weight as described above.

The feed to the non-adiabatic crystallizer (i.e., secondary fraction109) can be processed in a manner similar to that described above forfeed solution 1 in FIG. 1. In the operation of the non-adiabaticevaporative crystallizer 125, heat is transferred to secondary fraction109 to vaporize water (and small molecule impurities, such asformaldehyde and formic acid) and form a non-adiabatic crystallizeroverhead vapor stream 127. The N-(phosphonomethyl)glycine productprecipitates to produce the evaporative crystallization slurry 126comprising precipitated N-(phosphonomethyl)glycine product and secondarymother liquor containing dissolved N-(phosphonomethyl)glycine productand impurities. Slurry 126 is withdrawn from the non-adiabaticevaporative crystallizer 125, and divided into plural fractionscomprising a first fraction 129 and a second fraction 131. Firstfraction 129 is introduced into a first liquid/solids separation device133, preferably a solid bowl centrifuge, to produce a first fractionwet-cake product 153 having a solids content of from about 70% to about85% by weight as described above and a solids-depleted stream 134 (e.g.,centrate). The solids-depleted stream 134 is typically recycled back tothe non-adiabatic evaporative crystallizer 125. However, at least aportion of the solids-depleted stream 134 can optionally be mixed backwith the wet-cake as shown by the dashed line in FIG. 2 to generate afirst fraction wet-cake product 153A of even lower solids content. Thefirst fraction wet-cake product 153 or 153A is then preferably blendedwith the wet-cake product 119 produced from the adiabatic crystallizer111 to produce first wet-cake product 121. However, it should beunderstood that first fraction wet-cake product 153 or 153A and wet-cakeproduct 119 may be individually subjected to further processing withoutfirst combining these materials to produce first wet-cake product 121.Furthermore, at least a portion of the solids-depleted stream 134 canoptionally be blended with the first fraction wet-cake product 153 andthe wet-cake product 119 produced from the adiabatic crystallizer 111 toproduce first wet-cake product 121 as shown by the dashed line in FIG.2.

The second fraction 131 of the evaporative product slurry is optionallyintroduced into a hydroclone (or bank of hydroclones) 135 to form aconcentrated second slurry fraction 137 enriched in precipitatedN-(phosphonomethyl)glycine product and a solids-depleted stream 139. Thehydroclone solids-depleted stream 139 is preferably recycled back to theheat-driven evaporative crystallizer 125 for further recovery of theN-(phosphonomethyl)glycine product. The concentrated second fraction 137is introduced into a separator feed tank 141, which feeds a secondliquid/solids separation device, preferably a basket centrifuge capableof producing a wet-cake product having a relatively high solids content(typically from at least about 85% to about 99% by weight solids).Alternatively, the second fraction 131 of the evaporative product slurrycan be fed directly into the separator feed tank 141 or directly intothe second liquid/solids separation device. In the preferred embodimentshown in FIG. 2, the concentrated second slurry fraction 137 isintroduced into a bank of basket centrifuges operated in parallel. Thus,the concentrated slurry accumulated in the separator feed tank 141 isdivided into concentrated slurry fractions 143A and 143B that areintroduced into basket centrifuges 145A and 145B, respectively. Thebasket centrifuges each produce a product wet-cake 149A and 149B,respectively, that are combined to form a second wet-cake product 151.The basket centrifuges also produce centrates 147A and 147B that arefurther depleted in precipitated product and may be recycled back to thenon-adiabatic evaporative crystallizer 125. Alternatively, if necessaryto obtain a wet-cake product of acceptable purity, at least a portion ofcentrates 147A, 147B and/or 134 may be purged from the process. Itshould be understood that as described herein, a bank of liquid/solidsseparation devices are considered to be operated in parallel even thoughthe batch dewatering cycle in individual devices may not be in phase.

In operation of the product recovery system shown in FIG. 2, it will beexpected that the impurity concentration in the primary mother liquorgenerated in the adiabatic crystallizer system will be lower than theimpurity concentration in the secondary mother liquor generated in thenon-adiabatic evaporative crystallizer system, particularly because theratio of overheads to feed for the non-adiabatic crystallizer issignificantly larger than the ratio of overhead to feed for theadiabatic crystallizer. It is also expected that the second wet-cakeproduct 151, because of the lower amount of entrained mother liquor,will typically have an impurity level that meets specification andcontains at least about 95% by weight N-phosphonomethyl)glycine productdry basis. However, first fraction wet-cake product 153, without furtherprocessing, may not be of acceptable purity due to the increased amountof entrained mother liquor. By combining first fraction wet-cake product153 with wet-cake product 119 (generally a higher purity material), theoverall ratio of impurity to N-(phosphonomethyl)glycine can be madeacceptable, and thus further processing can generate a saleable productfrom this material. Such further processing may include drying to removeexcess water to generate a wet-cake or further addition of baseneutralization components to generate a suitableN-(phosphonomethyl)glycine salt product or formulation of acceptablepurity. For example, the N-(phosphonomethyl)glycine product in the firstwet-cake product 121, or in the first fraction wet-cake product 153 andwet-cake product 119 individually, may be neutralized with a base orbases in a conventional manner to prepare an agronomically acceptablesalt of N-(phosphonomethyl)glycine as is commonly used in glyphosateherbicidal formulations. Examples of agronomically acceptable salts ofN-(phosphonomethyl)glycine contain a cation selected from alkali metalcations (e.g., potassium and sodium ions), ammonium ion, isopropylammonium ion, tetra-alkylammonium ion, trialkyl sulfonium ion,protonated primary amine, protonated secondary amine and protonatedtertiary amine. Thus, the embodiment shown in FIG. 2 can readilygenerate at least two distinct products of acceptable purity, secondwet-cake product 151 and a product resulting from further processing offirst wet-cake product 121 and provide overall improved processflexibility.

Although the embodiment shown in FIG. 2 utilizing two or morecrystallization operations operated in semi-parallel has been found tobe advantageous for producing a plurality of acceptable wet-cakeproducts, depending on the incoming impurity levels in the oxidationreaction solution 105 or the fraction of second wet-cake product 151produced from the non-adiabatic crystallizer, there may be a limit tothe amount of second wet-cake product 151 having acceptable purity thatcan be made. In some instances, conventional washing of the secondwet-cake product 151 can be used to reduce impurity concentrations andincrease the amount of acceptable material 151 produced. However, insome instances described below, there are practical limits to the amountof cake washing that can be employed.

As the relative production of second wet-cake product 151 increases,impurities tend to accumulate in the secondary mother liquor of thenon-adiabatic crystallizer system to a point where concentrations aresufficiently high to significantly undermine washing efficiency.Increased impurity concentrations tend to decrease crystal size suchthat subsequent dewatering operations are hampered and significantquantities of impurity-containing liquid remain entrained in secondwet-cake product 151. Furthermore, it is believed that at higherconcentrations, some of these impurities may become incorporated intothe product crystals, decreasing cake-washing efficiency. These “solidphase occluded impurities” or other difficult to remove impurities insecond wet-cake product 151 may require extensive washing of thecrystals or other rigorous measures such as reslurrying with water orrecrystallization in order to meet typical product purityspecifications. These washes are usually recycled back to theevaporative crystallizer 125 to minimize the loss of soluble product.Unfortunately, the washed impurities also recycle and concentrate in theevaporative crystallizer, exacerbating the solid phase impurityocclusion problem and can also concentrate corrosive compounds raisingmaterials of construction concerns, and ultimately leading to centratepurges (e.g., 147A, 147B and/or 134). The impurities not purged in thecentrate end up in second wet-cake product 151, resulting in adisproportionate impurity redistribution to this portion of the product.In any event, as the amount of wash water increases, it becomesimpractical and cost prohibitive to evaporate the recycled washes in theevaporative crystallizer, nor can these washes be recycled to otheroperations in the process or purged from the process without raisingother concerns with respect to product purity and overall processefficiency.

In accordance with a further embodiment of the present invention, it hasbeen discovered that the limitations described above may be overcome,increased process flexibility obtained and better impurity managementachieved if the wet-cake products produced are the result of blendingmaterial from one of the crystallization operations with material fromthe other crystallization operation and preferably when material istransferred from the adiabatic crystallizer system into thenon-adiabatic crystallizer system. This increased process flexibility isparticularly useful as higher fractions of the total production aredirected to the production of second wet-cake product 151. Moreparticularly, it has been found that impurities within the wet-cakeproducts produced by the process of the present invention may bemaintained below desired levels by: (i) net transfer of impuritiescontained in the first (i.e., primary) and/or second (i.e., secondary)mother liquor fractions to the other of the first (i.e., adiabatic) andsecond (i.e., non-adiabatic evaporative) crystallization operations;(ii) net transfer of impurities contained in the first and/or secondmother liquor fractions to the other of the first and secondliquid/solids separation steps associated with the other of the firstand second crystallization operations; (iii) net transfer of wet-cakeproduct of relatively low impurities content, as obtained from one ofthe first and second liquid/solids separation steps, to the other of thefirst and second crystallization operations; (iv) net transfer ofwet-cake product of relatively low impurities content, as obtained fromone of the first and second liquid/solids separation steps, to the otherof the first and second liquid/solids separation steps associated withthe other of the first and second crystallization operations; (v) nettransfer of product slurry or magma of relatively low impuritiescontent, as obtained in one of the first and second crystallizationoperations, to the other of the first and second crystallizationoperations; (vi) net transfer of product slurry or magma of relativelylow impurities content, as obtained in one of the first and secondcrystallization operations, to the other of the first and secondliquid/solids separation steps associated with the other of the firstand second crystallization operations; or any combination of (i), (ii),(iii), (iv), (v) and/or (vi).

A preferred embodiment of the process of the present invention forproducing and recovering two wet-cake products comprising crystallineN-(phosphonomethyl)glycine from an oxidation reaction solutioncomprising dissolved N-(phosphonomethyl)glycine product and impuritiesis shown in FIG. 3. Similar to the process shown and described in FIG.2, the product recovery system of FIG. 3 employs a combination of anadiabatic crystallizer system and a non-adiabatic heat-drivenevaporative crystallizer operated in semi-parallel. However, inaccordance with this embodiment, impurity distribution among thecrystalline N-(phosphonomethyl)glycine wet-cake products is managed bynet transfer of primary product slurry or magma from the adiabaticcrystallizer system and combining it with N-(phosphonomethyl)glycineproduct contained in the secondary fraction of the oxidation reactionsolution. More particularly, in the embodiment illustrated in FIG. 3,impurity distribution among the crystalline N-(phosphonomethyl)glycinewet-cake products is managed by net transfer of primary product slurryto the secondary product slurry or magma of the evaporativecrystallizer.

Many of the various streams shown in FIG. 3 are analogous to thosedescribed above with respect to FIG. 2. Referring now to FIG. 3, anaqueous feed stream 101 comprising an N-(phosphonomethyl)iminodiaceticacid substrate is introduced along with oxygen into an oxidation reactorsystem 103 comprising one or more oxidation reaction zone(s), whereinthe N-(phosphonomethyl)iminodiacetic acid substrate is oxidativelycleaved in the presence of a catalyst to form an oxidation reactionsolution 105. The oxidation reaction solution 105 withdrawn from thereactor system 103 is then divided into plural fractions and a portion107 (i.e., a primary fraction of the oxidation reaction solution) isintroduced into an adiabatic crystallizer 111 to produce a primaryproduct slurry 113 comprising precipitated N-(phosphonomethyl)glycineproduct crystals and primary mother liquor. Another portion 109 (i.e., asecondary fraction of the oxidation reaction solution) is introducedinto a non-adiabatic heat-driven evaporative crystallizer 125 to producean evaporative crystallization slurry 126 (i.e., a secondary productslurry) comprising precipitated N-(phosphonomethyl)glycine productcrystals and secondary mother liquor.

Operation of the adiabatic crystallizer 111 produces vapor 115 (i.e.,the adiabatic crystallizer overhead) discharged from the top of thecrystallizer, a decantate (i.e., primary mother liquor) stream 112withdrawn from the crystallizer and a primary crystallization productslurry 113 removed from the bottom of the crystallizer and comprisingprecipitated crystalline N-(phosphonomethyl)glycine product and primarymother liquor. Preferably, at least a portion (and more preferably all)of the adiabatic crystallizer overhead 115 and/or decantate 112withdrawn from the adiabatic crystallizer 111 is/are recycled back tothe oxidation reactor system 103.

The primary crystallization product slurry 113 is divided into twoportions 113A and 113B. Portion 113A is introduced into a liquid/solidsseparation device 117, preferably a basket centrifuge or bank of basketcentrifuges, to produce a wet-cake product 119 and a solids-depletedstream 123 (e.g., centrate). At least a portion of the solids-depletedstream 123 may be recycled back to the adiabatic crystallizer 111 and/oroptionally may be recycled back to the oxidation reactor system 103 asshown by the dashed line in FIG. 3. Preferably, the wet-cake product 119has a solids content of from about 90% to about 99% by weight asdescribed above. Portion 113B is transferred to separator feed tank 141as described below.

In the operation of the non-adiabatic evaporative crystallizer 125, heatis transferred to secondary fraction 109 to vaporize water (and smallmolecule impurities, such as formaldehyde and formic acid) and form anon-adiabatic crystallizer overhead vapor stream 127. TheN-(phosphonomethyl)glycine product precipitates to produce anevaporative crystallization slurry 126 comprising precipitatedcrystalline N-(phosphonomethyl)glycine product and secondary motherliquor. Slurry 126 is withdrawn from the non-adiabatic evaporativecrystallizer 125, and divided into plural fractions comprising a firstfraction 129 and a second fraction 131. First fraction 129 is introducedinto a first liquid/solids separation device 133, preferably a solidbowl centrifuge, to produce a first fraction wet-cake product 153 havinga solids content of from about 70% to about 85% by weight as describedabove and a solids-depleted stream 134 (e.g., centrate). Thesolids-depleted stream 134 is typically recycled back to thenon-adiabatic evaporative crystallizer 125. However, at least a portionof the solids-depleted stream 134 can optionally be mixed back with thewet-cake as shown by the dashed line in FIG. 3 to generate a firstfraction wet-cake product 153A of even lower solids content. The firstfraction wet-cake product 153 or 153A is then preferably blended withthe wet-cake product 119 produced from the adiabatic crystallizer 111described above to produce first wet-cake product 121.

The second fraction 131 of the evaporative product slurry is optionallyintroduced into a hydroclone (or bank of hydroclones) 135 to form aconcentrated second slurry fraction 137 enriched in precipitatedN-(phosphonomethyl)glycine product and a solids-depleted stream 139. Thehydroclone solids-depleted stream 139 is preferably recycled back to theheat-driven evaporative crystallizer 125 for further recovery of theN-(phosphonomethyl)glycine product. The concentrated second fraction 137is introduced into a separator feed tank 141 and combined with portion113B of the primary product slurry to form a secondary fraction productmixture 143. Secondary fraction product mixture 143 is fed to aliquid/solids separation device, preferably a basket centrifuge capableof producing a wet-cake product having a relatively high solids content(typically from at least about 85% to about 99% by weight solids).Alternatively, the second fraction 131 of the evaporative product slurrycan be fed directly into the separator feed tank 141 or both the secondfraction 131 of the evaporative product slurry and the portion 113B ofthe primary product slurry can be fed directly into the secondliquid/solids separation device. In the preferred embodiment shown inFIG. 3, the secondary fraction product mixture 143 is introduced into abank of basket centrifuges operated in parallel. Thus, the secondaryfraction product mixture 143 from the separator feed tank 141 is dividedinto product mixture fractions 143A and 143B that are introduced intobasket centrifuges 145A and 145B, respectively. The basket centrifugeseach produce a product wet-cake 149A and 149B that are combined to formthe second wet-cake product 151. The basket centrifuges further producecentrates 147A and 147B that are further depleted in precipitatedproduct and may be recycled back to the non-adiabatic evaporativecrystallizer 125. Alternatively, if necessary to obtain a wet-cakeproduct of acceptable purity, at least a portion of centrates 147A, 147Band/or 134 may be purged from the process.

Operation of the embodiment shown in FIG. 3 is particularly advantageousto resolve the limitations on production of second wet-cake product 151(relative to the total system production) imposed by the operation ofthe system shown in FIG. 2 when cake washing of second wet-cake product151 becomes impractical. The solid and liquid phase impurities in theportion 113B of primary product slurry 113 are considerably lower thanthose in the concentrated second slurry fraction 137. The blending ofthese streams in 143 above a minimum ratio lowers the average impuritylevel in the solid and/or liquid phase, enabling a decrease andultimately elimination of the water wash of second wet-cake product 151.The resulting second wet-cake product 151 may carry a greater quantityof impurities from the evaporative crystallizer than otherwise,resulting in a better balance of impurities between wet-cake products121 and 151. This occurs in spite of the partial dilution of liquidphase impurities in second product fraction mixture 143 due to the lowerimpurity content in portion 113B of primary product slurry 113. Intypical practice, advantageous results are achieved when from about 10%to about 30% by weight of the primary product slurry 113 is transferredto secondary fraction product mixture 143. However, it should beunderstood that the exact proportion can vary considerably withoutdeparting from the scope of the present invention and, as would beunderstood by those skilled in the art, is dependent upon a variety ofparameters, including the composition of the secondary product slurry126 from the evaporative crystallizer.

Other preferred embodiments for producing and recovering two wet-cakeproducts comprising crystalline N-(phosphonomethyl)glycine from anoxidation reaction solution comprising dissolvedN-(phosphonomethyl)glycine product and impurities are shown in FIGS.4-6. Similar to the processes shown and described in FIGS. 2 and 3, theproduct recovery systems of these additional embodiments employ acombination of an adiabatic crystallizer system and a non-adiabaticheat-driven evaporative crystallizer operated in semi-parallel.Accordingly, many of the various streams shown in FIGS. 4-6 areanalogous to those described above with respect to FIGS. 2 and 3.

The process embodiment illustrated in FIG. 4 is a variation of theprocess described in FIG. 3 in which impurity distribution among thecrystalline N-(phosphonomethyl)glycine wet-cake products is likewisemanaged by net transfer of primary product slurry or magma from theadiabatic crystallizer system and combining it withN-(phosphonomethyl)glycine product contained in the secondary fractionof the oxidation reaction solution. However, in the process depicted inFIG. 4, impurity distribution among the crystallineN-(phosphonomethyl)glycine wet-cake products is managed by net transferof primary product slurry or magma from the adiabatic crystallizersystem to the evaporative crystallizer.

Referring now to FIG. 4, an aqueous feed stream 101 comprising anN-(phosphonomethyl)iminodiacetic acid substrate is introduced along withoxygen into an oxidation reactor system 103 comprising one or moreoxidation reaction zone(s), wherein the N-(phosphonomethyl)iminodiaceticacid substrate is oxidatively cleaved in the presence of a catalyst toform an oxidation reaction solution 105. The oxidation reaction solution105 withdrawn from the reactor system 103 is then divided into pluralfractions and a portion 107 (i.e., a primary fraction of the oxidationreaction solution) is introduced into an adiabatic crystallizer 111 toproduce a primary product slurry 113 comprising precipitatedN-(phosphonomethyl)glycine product crystals and primary mother liquor.Another portion 109 (i.e., a secondary fraction of the oxidationreaction solution) is introduced into a non-adiabatic heat-drivenevaporative crystallizer 125 to produce an evaporative crystallizationslurry 126 (i.e., a secondary product slurry) comprising precipitatedN-(phosphonomethyl)glycine product crystals and secondary mother liquor.

Operation of the adiabatic crystallizer 111 produces vapor 115 (i.e.,the adiabatic crystallizer overhead) discharged from the top of thecrystallizer, a decantate (i.e., primary mother liquor) stream 112withdrawn from the crystallizer and a primary crystallization productslurry 113 removed from the bottom of the crystallizer and comprisingprecipitated crystalline N-(phosphonomethyl)glycine product and primarymother liquor. Preferably, at least a portion (and more preferably all)of the adiabatic crystallizer overhead 115 and/or decantate 112withdrawn from the adiabatic crystallizer 111 is/are recycled back tothe oxidation reactor system 103.

The primary crystallization product slurry 113 is divided into twoportions 113A and 113B. Portion 113A is introduced into a liquid/solidsseparation device 117, preferably a basket centrifuge or bank of basketcentrifuges, to produce a wet-cake product 119 and a solid-depletedstream 123 (e.g., centrate). At least a portion of the solids-depletedstream 123 may be recycled back to the adiabatic crystallizer 111 and/oroptionally may be recycled back to the oxidation reactor system 103 asshown by the dashed line in FIG. 4. Preferably, the wet-cake product 119has a solids content of from about 90% to about 99% by weight asdescribed above.

Portion 113B of the primary product slurry 113 is combined with thesecondary fraction 109 of the oxidation reaction solution to form anevaporative crystallizer feed mixture transferred to the evaporativecrystallizer 125 for precipitation of crystallineN-(phosphonomethyl)glycine product. While not necessary or critical tothe present invention, it is contemplated that portion 113B may beintroduced directly into the evaporative crystallizer 125 or may bepremixed with secondary fraction 109, for example, in a holding tank(not shown). In either case, heat is transferred to the resultingevaporative crystallizer feed mixture in the non-adiabatic evaporativecrystallizer 125 to vaporize water (and small molecule impurities, suchas formaldehyde and formic acid) and form a non-adiabatic crystallizeroverhead vapor stream 127. The N-(phosphonomethyl)glycine productprecipitates to produce an evaporative crystallization slurry 126comprising precipitated crystalline N-(phosphonomethyl)glycine productand secondary mother liquor. Slurry 126 is withdrawn from thenon-adiabatic evaporative crystallizer 125, and divided into pluralfractions comprising a first fraction 129 and a second fraction 131.First fraction 129 is introduced into a first liquid/solids separationdevice 133, preferably a solid bowl centrifuge, to produce a firstfraction wet-cake product 153 having a solids content of from about 70%to about 85% by weight as described above and a solids-depleted stream134 (e.g., centrate). The solids-depleted stream 134 is typicallyrecycled back to the non-adiabatic evaporative crystallizer 125.However, at least a portion of the solids-depleted stream 134 canoptionally be mixed back with the wet-cake as shown by the dashed linein FIG. 4 to generate a first fraction wet-cake product 153A of evenlower solids content. The first fraction wet-cake product 153 or 153A isthen preferably blended with the wet-cake product 119 produced from theadiabatic crystallizer 111 described above to produce first wet-cakeproduct 121.

The second fraction 131 of the evaporative product slurry is optionallyintroduced into a hydroclone (or bank of hydroclones) 135 to form aconcentrated second slurry fraction 137 enriched in precipitatedN-(phosphonomethyl)glycine product and a solids-depleted stream 139. Thehydroclone solids-depleted stream 139 is preferably recycled back to theheat-driven evaporative crystallizer 125 for further recovery of theN-(phosphonomethyl)glycine product. The concentrated second fraction 137is introduced into a separator feed tank 141, which feeds a secondliquid/solids separation device, preferably a basket centrifuge capableof producing a wet-cake product having a relatively high solids content(typically from at least about 85% to about 99% by weight solids).Alternatively, the second fraction 131 of the evaporative product slurrycan be fed directly into the separator feed tank 141 or directly intothe second liquid/solids separation device. In the preferred embodimentshown in FIG. 4, the concentrated second slurry fraction 137 isintroduced into a series of basket centrifuges operated in parallel.Thus, the concentrated slurry accumulated in the separator feed tank 141is divided into concentrated slurry fractions 143A and 143B that areintroduced into basket centrifuges 145A and 145B, respectively. Thebasket centrifuges each produce a product wet-cake 149A and 149B thatare combined to form the second product wet-cake 151. The basketcentrifuges further produce centrates 147A and 147B that are furtherdepleted in precipitated product and may be recycled back to thenon-adiabatic evaporative crystallizer 125. Alternatively, if necessaryto obtain a wet-cake product of acceptable purity, at least a portion ofcentrates 147A, 147B and/or 134 may be purged from the process.

Without being bound to a particular theory, it is believed that transferof the portion 113B of primary product slurry 113 to the evaporativecrystallizer may advantageously affect the precipitation ofN-(phosphonomethyl)glycine product crystals from secondary fraction 109of the oxidation reaction solution such that less impurities and bettercrystal size distribution may be obtained. More particularly, portion113B of the adiabatic primary product slurry typically contains largeproduct crystals of high purity. Thus, transfer of portion 113B to theevaporative crystallizer may effectively “seed” the crystallizer topromote crystal growth such that less impurities may be incorporated inthe crystal structure. In any case, one skilled in the art wouldunderstand that any crystal growth incorporating the relatively purecrystals from portion 113B will increase the overall purity profile ofthe product slurry produced from the evaporative crystallizationoperation.

In practice, the proportion of the primary product slurry 113transferred to the non-adiabatic evaporative crystallizer 125 can varyconsiderably without departing from the scope of the present inventionand advantageous results obtained.

Another preferred embodiment of the present invention for producing andrecovering two wet-cake products comprising crystallineN-(phosphonomethyl)glycine product from an oxidation reaction solutioncomprising dissolved N-(phosphonomethyl)glycine and impurities is shownin FIG. 5. In accordance with this further embodiment, impuritydistribution among the crystalline N-(phosphonomethyl)glycine wet-cakeproducts is managed by net transfer of N-(phosphonomethyl)glycineproduct crystals contained in the first wet-cake product from theadiabatic crystallizer system and combining the crystals withN-(phosphonomethyl)glycine product contained in the secondary fractionof the oxidation reaction solution. More particularly, in the embodimentillustrated in FIG. 5, impurity distribution among the crystallineN-(phosphonomethyl)glycine wet-cake products is managed by net transferof N-(phosphonomethyl)glycine product crystals from the first wet-cakeproduct to the secondary product slurry or magma of the evaporativecrystallizer.

Referring now to FIG. 5, an aqueous feed stream 101 comprising anN-(phosphonomethyl)iminodiacetic acid substrate is introduced along withoxygen into an oxidation reactor system 103 comprising one or moreoxidation reaction zone(s), wherein the N-(phosphonomethyl)iminodiaceticacid substrate is oxidatively cleaved in the presence of a catalyst toform an oxidation reaction solution 105. The oxidation reaction solution105 withdrawn from the reactor system 103 is then divided into pluralfractions and a portion 107 (i.e., a primary fraction of the oxidationreaction solution) is introduced into an adiabatic crystallizer 111 toproduce a primary product slurry 113 comprising precipitatedN-(phosphonomethyl)glycine product crystals and primary mother liquor.Another portion 109 (i.e., a secondary fraction of the oxidationreaction solution) is introduced into a non-adiabatic heat-drivenevaporative crystallizer 125 to produce an evaporative crystallizationslurry 126 (i.e., a secondary product slurry) comprising precipitatedN-(phosphonomethyl)glycine product crystals and secondary mother liquor.

Operation of the adiabatic crystallizer 111 produces vapor 115 (i.e.,the adiabatic crystallizer overhead) discharged from the top of thecrystallizer, a decantate (i.e., primary mother liquor) stream 112withdrawn from the crystallizer and a primary crystallization productslurry 113 removed from the bottom of the crystallizer and comprisingprecipitated crystalline N-(phosphonomethyl)glycine product and primarymother liquor. Preferably, at least a portion (and more preferably all)of the adiabatic crystallizer overhead 115 and/or decantate 112withdrawn from the adiabatic crystallizer 111 is/are recycled back tothe oxidation reactor system 103.

The primary crystallization product slurry 113 comprising precipitatedcrystalline N-(phosphonomethyl)glycine product and primary mother liquorremoved from the bottom of the crystallizer is introduced into aliquid/solids separation device 117, preferably a basket centrifuge orbank of basket centrifuges, to produce a primary wet-cake product 119and a solids-depleted stream 123 (e.g., centrate). At least a portion ofthe solids-depleted stream 123 may be recycled back to the adiabaticcrystallizer 111 and/or optionally may be recycled back to the oxidationreactor system as shown by the dashed line in FIG. 5. Preferably, theprimary wet-cake product 119 has a solids content of from about 90% toabout 99% by weight as described above. As further described below, atleast a portion 119B of the primary wet-cake 119 is transferred toseparator feed tank 141 for blending with the second fraction productslurry produced in the evaporative crystallization operation.Preferably, another portion 119A of the primary wet-cake 119 is reservedfor inclusion in first wet-cake product 121.

In the operation of the non-adiabatic evaporative crystallizer, heat istransferred to secondary fraction 109 to vaporize water (and smallmolecule impurities, such as formaldehyde and formic acid) and form anon-adiabatic crystallizer overhead vapor stream 127. TheN-(phosphonomethyl)glycine product precipitates to produce anevaporative crystallization slurry 126 comprising precipitatedcrystalline N-(phosphonomethyl)glycine product and secondary motherliquor. Slurry 126 is withdrawn from the non-adiabatic evaporativecrystallizer 125, and divided into plural fractions comprising a firstfraction 129 and a second fraction 131. First fraction 129 is introducedinto a first liquid/solids separation device 133, preferably a solidbowl centrifuge, to produce a first fraction wet-cake product 153 havinga solids content of from about 70% to about 85% by weight as describedabove and a solids-depleted stream 134 (e.g., centrate). Thesolids-depleted stream 134 is typically recycled back to thenon-adiabatic evaporative crystallizer 125. However, at least a portionof the solids-depleted stream 134 can optionally be mixed back with thewet-cake as shown by the dashed line in FIG. 5 to generate a firstfraction wet-cake product 153A of even lower solids content. The firstfraction wet-cake product 153 or 153A is then preferably blended withportion 119A of the wet-cake product 119 produced from the adiabaticcrystallizer 111 to produce first wet-cake product 121.

The second fraction 131 of the evaporative product slurry is optionallyintroduced into a hydroclone (or bank of hydroclones) 135 to form aconcentrated second slurry fraction 137 enriched in precipitatedN-(phosphonomethyl)glycine product and a solids-depleted stream 139. Thehydroclone solids-depleted stream 139 is preferably recycled back to theheat-driven evaporative crystallizer 125 for further recovery of theN-(phosphonomethyl)glycine product. The concentrated second fraction 137is introduced into a separator feed tank 141 and combined with portion119B of the wet-cake produced in the adiabatic crystallization operationdescribed above to form a secondary fraction product mixture 143.Secondary fraction product mixture 143 is fed into a secondliquid/solids separation device, preferably a basket centrifuge capableof producing a wet-cake product having a relatively high solids content(typically from at least about 85% to about 99% by weight solids).Alternatively, the second fraction 131 of the evaporative product slurrycan be fed directly into the separator feed tank 141 or both the secondfraction 131 of the evaporative product slurry and the portion 119B ofthe wet-cake produced in the adiabatic crystallization operation can befed directly into the second liquid/solids separation device. In thepreferred embodiment shown in FIG. 5, the secondary fraction productmixture 143 is introduced into a bank of basket centrifuges operated inparallel. Thus, the secondary fraction product mixture 143 from theseparator feed tank 141 is divided into product mixture fractions 143Aand 143B that are introduced into basket centrifuges 145A and 145B,respectively. The basket centrifuges each produce a product wet-cake149A and 149B that are combined to form the second wet-cake product 151.The basket centrifuges further produce centrates 147A and 147B that arefurther depleted in precipitated product and may be recycled back to thenon-adiabatic evaporative crystallizer 125. Alternatively, if necessaryto obtain a wet-cake product of acceptable purity, at least a portion ofcentrates 147A, 147B and/or 134 may be purged from the process.

As compared to FIG. 3, the blending of adiabatic wet-cake in secondaryfraction product mixture 143 instead of adiabatic slurry allows a higherlevel of liquid phase impurities to be carried by the combined solidsstream, resulting in an improvement in impurity distribution away fromfirst wet-cake product 121. This scheme also reduces the evaporationload in the evaporative crystallizer 125 by the reduction in waterflowing into the evaporative crystallizer system.

In a further process embodiment of the present invention in whichimpurity distribution among the crystalline N-(phosphonomethyl)glycinewet-cake products is managed by net transfer ofN-(phosphonomethyl)glycine product crystals contained in the firstwet-cake product from the adiabatic crystallizer system and combiningthe crystals with N-(phosphonomethyl)glycine product contained in thesecondary fraction of the oxidation reaction solution, the processillustrated in FIG. 5 is modified such that N-(phosphonomethyl)glycineproduct crystals from the first wet-cake product are transferred to theevaporative crystallizer. That is, portion 119B of the primary wet-cake119 is combined with the secondary fraction 109 of the oxidationreaction solution to form an evaporative crystallizer feed mixturetransferred to the evaporative crystallizer 125 for precipitation ofcrystalline N-(phosphonomethyl)glycine product. While not necessary orcritical to the present invention, it is contemplated that portion 119Bmay be introduced directly into the evaporative crystallizer 125 or maybe premixed with secondary fraction 109, for example, in a holding tank.

In practice, the proportion of the adiabatic wet-cake 119 transferred tosecondary fraction product mixture 143 and/or to the evaporativecrystallizer 125 can vary considerably without departing from the scopeof the present invention and advantageous results obtained.

In a still further alternative embodiment of the process illustrated inFIG. 5, rather than blending adiabatic wet-cake 119B in secondaryfraction product mixture 143 and/or introducing it into the evaporativecrystallizer 125, adiabatic wet-cake may be combined and physicallymixed directly with the second wet-cake product 151 to obtain a combinedwet-cake product of acceptable purity.

A still further embodiment of the process of the present invention forproducing and recovering two wet-cake products comprising crystallineN-(phosphonomethyl)glycine product from an oxidation reaction solutioncomprising dissolved N-(phosphonomethyl)glycine product and impuritiesis shown in FIG. 6. In this embodiment, impurity distribution among thecrystalline N-(phosphonomethyl)glycine wet-cake products is managed bynet transfer of impurities contained in one of the first and secondmother liquor fractions to: (i) the other of the first and secondcrystallization operations; (ii) the other of the first and secondliquid/solids separation steps; (iii) the other of the first and secondwet-cake products; or any combination of (i), (ii) and/or (iii).

Referring now to FIG. 6, an aqueous feed stream 101 comprising anN-(phosphonomethyl)iminodiacetic acid substrate is introduced along withoxygen into an oxidation reactor system 103 comprising one or moreoxidation reaction zone(s), wherein the N-(phosphonomethyl)iminodiaceticacid substrate is oxidatively cleaved in the presence of a catalyst toform an oxidation reaction solution 105. The oxidation reaction solution105 withdrawn from the reactor system 103 is then divided into pluralfractions and a portion 107 (i.e., a primary fraction of the oxidationreaction solution) is introduced into an adiabatic crystallizer 111 toproduce a primary product slurry 113 comprising precipitatedN-(phosphonomethyl)glycine product crystals and primary mother liquor.Another portion 109 (i.e., a secondary fraction of the oxidationreaction solution) is introduced into a non-adiabatic heat-drivenevaporative crystallizer 125 to produce an evaporative crystallizationslurry 126 (i.e., a secondary product slurry) comprising precipitatedN-(phosphonomethyl)glycine product crystals and secondary mother liquor.

Operation of the adiabatic crystallizer 111 produces vapor 115 (i.e.,the adiabatic crystallizer overhead) discharged from the top of thecrystallizer, a decantate (i.e., primary mother liquor) stream 112withdrawn from the crystallizer and a primary crystallization productslurry 113 removed from the bottom of the crystallizer and comprisingprecipitated crystalline N-(phosphonomethyl)glycine product and primarymother liquor. Preferably, at least a portion (and more preferably all)of the adiabatic crystallizer overhead 115 and/or decantate 112withdrawn from the adiabatic crystallizer 111 is/are recycled back tothe oxidation reactor system 103.

The primary crystallization product slurry 113 comprising precipitatedcrystalline N-(phosphonomethyl)glycine product and primary mother liquorremoved from the bottom of the adiabatic crystallizer is introduced intoa liquid/solids separation device 117, preferably a basket centrifuge orbank of basket centrifuges, to produce a wet-cake product 119 and asolids-depleted stream 123 (e.g., centrate). At least a portion of thesolids-depleted stream 123 may be recycled back to the adiabaticcrystallizer 111 and/or optionally may be recycled back to the oxidationreactor system 103 as shown by the dashed line in FIG. 6. Preferably,the wet-cake product 119 has a solids content of from about 90% to about99% by weight as described above.

In the operation of the non-adiabatic evaporative crystallizer 125, heatis transferred to secondary fraction 109 to vaporize water (and smallmolecule impurities, such as formaldehyde and formic acid) and form anon-adiabatic crystallizer overhead vapor stream 127. TheN-(phosphonomethyl)glycine product precipitates to produce anevaporative crystallization slurry 126 comprising precipitatedcrystalline N-(phosphonomethyl)glycine product and secondary motherliquor. Slurry 126 is withdrawn from the non-adiabatic evaporativecrystallizer 125, and divided into plural fractions comprising a firstfraction 129 and a second fraction 131. First fraction 129 is introducedinto a first liquid/solids separation device 133, preferably a solidbowl centrifuge, to produce a first fraction wet-cake product 153 havinga solids content of from about 70% to about 85% by weight as describedabove and a solids-depleted stream 134 (e.g., centrate). Thesolids-depleted stream is typically recycled back to the non-adiabaticevaporative crystallizer. However, at least a portion of thesolids-depleted stream 134 can optionally be mixed back with thewet-cake as shown by the dashed line in FIG. 6 to generate a firstfraction wet-cake product 153A of even lower solids content. The firstfraction wet-cake product 153 or 153A is then preferably blended withthe wet-cake product 119 produced from the adiabatic crystallizer toproduce first wet-cake product 121.

The second fraction 131 of the evaporative product slurry is optionallyintroduced into a hydroclone (or bank of hydroclones) 135 to form aconcentrated second slurry fraction 137 enriched in precipitatedN-(phosphonomethyl)glycine product and a solids-depleted stream 139. Thehydroclone solids-depleted stream 139 is preferably recycled back to theheat-driven evaporative crystallizer 125 for further recovery of theN-(phosphonomethyl)glycine product. The concentrated second fraction 137is introduced into a separator feed tank 141, which feeds aliquid/solids separation device, preferably a basket centrifuge capableof producing a wet-cake product having a relatively high solids content(typically from at least about 85% to about 99% by weight solids). Inthe preferred embodiment shown in FIG. 6, the concentrated second slurryfraction 137 is introduced into a bank of basket centrifuges operated inparallel. Thus, the concentrated slurry accumulated in the separatorfeed tank 141 is divided into concentrated slurry fractions 143A and143B that are introduced into basket centrifuges 145A and 145B,respectively. The basket centrifuges each produce a product wet-cake149A and 149B that are combined to form the second wet-cake product 151.The basket centrifuges further produce centrates 147A and 147B that arefurther depleted in precipitated product and may be recycled back to thenon-adiabatic evaporative crystallizer 125. Alternatively, if necessaryto obtain a wet-cake product of acceptable purity, at least a portion ofcentrates 147A, 147B and/or 134 may be purged from the process.

As shown in FIG. 6, at least some options for management of impuritydistribution by net transfer of impurities contained in mother liquormay include, without limitation: transfer of primary mother liquor fromthe adiabatic crystallization (e.g., decantate 112 and/or thesolids-depleted stream 123) to evaporative crystallization operation125; transfer of centrates 147A and/or 147B to first wet-cake product121; transfer of centrates 147A and/or 147B to adiabatic crystallizer111; and/or transfer of centrates 147A and/or 147B to primary productslurry 113.

It is expected that the N-(phosphonomethyl)glycine product crystalsgenerated in the adiabatic crystallizer system will be larger. Thisprovides a material having good handling characteristics when blendedwith wet-cake from the non-adiabatic crystallizer system. However, theblended wet-cake might not allow for as much entrained liquid. Thereforeit might be desired to grind the adiabatic crystals to make a smallercrystal size, either to give a more uniform crystal size distributionwithin the blended material or to ensure a suitable amount of entrainedliquid with the blended wet-cake for impurity balance reasons.

EXAMPLES

The following examples are simply intended to further illustrate andexplain the present invention. The invention, therefore, should not belimited to any of the details in these examples.

Example 1

A sample of N-(phosphonomethyl)glycine wet-cake was submitted foranalysis and subsequent testing. The wet-cake was obtained from anon-adiabatic evaporative crystallizer stage used to dewater a productslurry obtained from catalytic oxidation ofN-(phosphonomethyl)iminodiacetic acid and was subjected to a subsequentcentrifuge wash cycle. The dried sample was analyzed for impurities,namely, formaldehyde, formic acid, N-methyl-N-(phosphonomethyl)glycine(NMG), aminomethylphosphonic acid (AMPA), methyl aminomethylphosphonicacid (MAMPA), iminodiacetic acid (IDA), glycine,imino-bis-(methylene)-bis-phosphonic acid (iminobis) andN-(phosphonomethyl)iminodiacetic acid (GI) and also forN-(phosphonomethyl)glycine content.

The sample was then separated into 3 separate equal weight fractions,and each of these was reslurried into room temperature water at 3different mass ratios—3:1, 7:1, and 15.67:1 water to dry solid. Theseratios are 1 to 2 order of magnitude higher than the typical waterdisplacement ratios during a centrifuge wash step, but insufficient tofully dissolve the solids. After a length of time, the solid sampleswere filtered, dried, and re-submitted for analysis.

The intent of the test was to find which impurities could be washed offthe solids and which were “occluded” as solid phase impurities. Table 1below shows each of the residual solid phase impurities that could notbe washed away by the re-pulp water washes. The other impurities in thewet-cake were believed to be washed away during the re-pulping. The datais shown in units of residual ppm impurity per weight percentN-(phosphonomethyl)glycine (Gly) in the solid phase. Only minimal purityincreases could be achieved by this method.

TABLE 1 Water/Solid GI/Gly Iminobis/Gly MAMPA/Gly AMPA/Gly IDA/GlyGlycine/Gly Ratio Ratio Ratio ppm/% Ratio ppm/% Ratio ppm/% Ratio ppm/%Ratio ppm/% ppm/% 0 21.4 110.6 46.9 87.0 22.5 13.8 3:1 20.6 98.1 41.776.5 16.5 9.5 7:1 20.7 97.4 41.3 75.2 17.6 9.4 15.67:1    20.9 94.9 41.972.7 16.8 8.9

The above solid phase impurities constituted between 256.1 ppm perweight percent N-(phosphonomethyl)glycine and 302.2 ppm per weightpercent N-(phosphonomethyl)glycine in the solid phase. To convert thesevalues to weight percent impurities in the dry wet-cake, the sum of thevalues in Table 1 for any given row is divided by 10,000 (this isdefined as X), and this resultant value is divided by 1-X. Performingthis arithmetic shows that an impurity level of 3.12 weight percent wascontained in the dry wet-cake before water re-pulp, and 2.63 weightpercent remained after much washing. It should be understood that thisimplies a N-(phosphonomethyl)glycine content of 96.9 to 97.4 weightpercent in the wet-cake in this example.

Example 2

An experiment was conducted using a system similar to that illustratedin FIG. 3 to produce and recover N-(phosphonomethyl)glycine wet-cakeproducts. Varying proportions 113B of the primary product slurry 113from the adiabatic crystallizer were blended with the fraction 137 ofthe evaporative crystallization slurry 126 (i.e., secondary productslurry) from the evaporative crystallizer in the evaporative centrifugefeed tank 141. During this experiment, the ratio of primary fraction 107to the oxidation reaction solution 105 averaged about 0.79, while theconcentration of N-(phosphonomethyl)glycine dissolved in solution 105averaged about 9% by weight. The solids content in primary productslurry 113 was maintained at about 25% by weight, while that insecondary product slurry 126 was maintained at about 11% by weight.

During the course of the experiment, the mass ratio of solids from theportion 113B of the primary product slurry to the combined solids in 143was varied from 0 to about 0.40 in increments of approximately 0.10.Initially, prior to blending material from the primary product slurry113 from the adiabatic crystallizer with the fraction 137 of thesecondary product slurry from the evaporative crystallizer, the fraction129 of the secondary product slurry fed to the solid bowl centrifuge 133averaged about 37% by weight, but as increased to about 55% by weight bythe end of the experiment, while still achieving the same wet-cakeproduction relative to total production (i.e., from about 15% to about16%). At each mass ratio of solids from the portion 113B of the primaryproduct slurry to the combined solids in 143, the amount of centrifugewash water was reduced until the N-(phosphonomethyl)glycine purity ofthe combined wet-cakes 149A and 149B matched that of the previous ratio.A ratio was ultimately reached where water wash was totally eliminated,yet the N-(phosphonomethyl)glycine assay of the combined wet-cakes 149Aand 149B exceeded that obtained prior to blending material from theprimary product slurry 113 from the adiabatic crystallizer with thefraction 137 of the secondary product slurry from the evaporativecrystallizer. The N-(phosphonomethyl)glycine assay in the combinedwet-cakes 149A and 149B went from 95.9 to 96.4% by weight (dry basis).

Example 3

A process material balance model was created and utilized to simulateand compare the product recovery systems as illustrated in FIGS. 2, 3and 5. All model simulations made the following assumptions and inputs.

The system is operated at steady-state and the same base feed amount ofN-(phosphonomethyl)iminodiacetic acid (GI) in aqueous feed stream 101 isfed to the oxidation reactor system 103. Feed water in aqueous feedstream 101 is adjusted to keep the N-(phosphonomethyl)glycine (Gly)concentration constant at 9.1% by weight in the reaction solution 105exiting the reactor system 103. The concentration of unreactedN-(phosphonomethyl)iminodiacetic acid in oxidation reaction solution 105is 900 ppm by weight. The selectivity of the oxidation reactor system103 is assumed to be such that 0.721 pounds ofN-(phosphonomethyl)glycine were formed for every pound ofN-(phosphonomethyl)iminodiacetic acid reacted. Additionally, theoxidation reactor system 103 is assumed to generate 0.00325 pounds ofimpurities for each pound of N-(phosphonomethyl)glycine formed in theoxidation reactor system. These impurities are assumed to benon-volatile in the process and remain in liquid or are occluded incrystal solids or co-crystallized.

The concentration of N-(phosphonomethyl)glycine in adiabaticcrystallizer primary mother liquor decantate 112 and centrate 123 is3.5% by weight. Solids concentration in primary adiabaticcrystallization product slurry 113 is assumed to be 25% by weight. Theratio of adiabatic crystallizer overhead 115 to primary fraction 107 ofthe oxidation reaction solution is assumed to be 0.07. The solidscontent of wet-cake product 119 is 92% by weight.

All liquid/solids separation devices employed in the process are assumedto generate solid-free liquids and the decantate from the adiabaticcrystallizer 111 is likewise solid-free.

The partition coefficient for N-(phosphonomethyl)iminodiacetic acidsolids in adiabatic crystallizer 111 is 0.90. TheN-(phosphonomethyl)iminodiacetic acid partition coefficient in theevaporative crystallizer 125 is 0.20. TheseN-(phosphonomethyl)iminodiacetic acid partition coefficients are definedas the ratio of the concentration of N-(phosphonomethyl)iminodiaceticacid in the solid to N-(phosphonomethyl)iminodiacetic acid in the liquidphase where the concentration in the solid phase is on aN-(phosphonomethyl)glycine-only basis. The impurities partitioncoefficient in evaporative crystallizer 125 with respect to non-volatileimpurities generated in the oxidation reactor system is 0.60. Thispartition coefficient is defined as the ratio of the concentration ofimpurities in the solid to impurities in the liquid phase where theimpurities concentration in the solid phase is on aN-(phosphonomethyl)glycine-only basis. The non-volatile impuritiespartition coefficient in the adiabatic crystallizer 111 is assumed to benegligible.

The solids content of secondary evaporative crystallization slurry 126is 15% by weight. Centrates 147A, 147B and 134 contain 7% by weightN-(phosphonomethyl)glycine.

The assumed ratio of solids concentration in concentrated slurry 137 tothe solids concentration in second fraction 131 of the secondaryevaporative crystallization slurry is 1.7.

The solids content of first fraction wet-cake product 153 is 70% byweight and the solids content of second weight cake product 151 is 88%by weight.

In model simulations, the calculations also assume 2500 ppm by weight ofvolatile reaction by-products present in oxidation reactor solution 105.These volatile components can leave in the overhead streams of theadiabatic crystallizer 111 and the evaporative crystallizer 125. Theconcentration of volatile impurities in the respective crystallizeroverheads is assumed to be equal to the concentration of volatileimpurities in all the materials fed to the adiabatic crystallizer 111and the evaporative crystallizer 125.

Case A: This is a simulated material balance for a process configurationsimilar to that illustrated in FIG. 2, with the second wet-cake product151 representing 31.11 weight percent of the totalN-(phosphonomethyl)glycine production. In this example, theN-(phosphonomethyl)glycine assay of the second weight cake product 151is 95.00% by weight (dry basis) and the required non-adiabaticevaporative crystallizer overhead 127 is 2.73 pounds per pound ofN-(phosphonomethyl)iminodiacetic acid feed to the reactor system 103.

Case B: This is a variation on Case A, but with the second wet-cake 151production increased to 35.57 weight percent of the totalN-(phosphonomethyl)glycine production, while keeping the overallrequired non-adiabatic evaporative crystallizer overhead 127 per poundof N-(phosphonomethyl)iminodiacetic acid feed to the reactor system 103approximately the same. In this case, the N-(phosphonomethyl)glycineassay of the second wet-cake product 151 drops to 93.91% by weight (drybasis). Case B illustrates a limit in the production rate of secondwet-cake product 151 having acceptable purity.

Case C: This is a further variation on Cases A and B except that thesecond wet-cake 151 production is further increased to 41.90% of thetotal N-(phosphonomethyl)glycine production and the feed water make-upin aqueous feed stream 101 is increased as more production is shifted tothe non-adiabatic crystallizer 125. In this case, theN-(phosphonomethyl)glycine assay of the second wet-cake product 151 issimilar to that as in Case A, but is realized at the cost of therequired non-adiabatic evaporative crystallizer overhead 127 increasingto 3.69 pounds per pound of N-(phosphonomethyl)iminodiacetic acid feedto the reactor system 103. Case C illustrates how a higher productionrate of second wet-cake product 151 can be achieved by shifting moreproduction to the non-adiabatic crystallizer, but at the expense ofincreased non-adiabatic evaporative crystallizer overhead 127requirements (i.e., increased non-adiabatic evaporative crystallizeroperating costs).

Cases A through C illustrate that increasing the percent of secondwet-cake product 151 in FIG. 2 comes at the expense of either reducedthe N-(phosphonomethyl)glycine assay of the second wet-cake product 151(Case B) and/or at the expense of additional non-adiabatic evaporativecrystallizer overhead 127 requirements (Case C).

Case D: This is a simulated material balance for a process configurationsimilar to that illustrated in FIG. 3 wherein a portion 113B of theprimary crystallization product slurry 113 from the adiabaticcrystallizer 111 is transferred and mixed with evaporativecrystallization slurry 126 from the non-adiabatic evaporativecrystallizer 125 in the evaporative crystallizer centrifuge feed tank141. In this case, 42% of the production can be produced as secondwet-cake product 151, while the N-(phosphonomethyl)glycine assay of thesecond wet-cake product 151 is higher than in Case A (which produced alower second wet-cake percentage) and non-adiabatic evaporativecrystallizer overhead 127 requirements are slightly reduced as comparedto Case A. This illustrates how mixing primary crystallization productslurry 113 from the adiabatic crystallizer with evaporativecrystallization slurry 126 from the non-adiabatic evaporativecrystallizer can yield increased second wet-cake 151 production withoutincreasing non-adiabatic evaporative crystallizer overhead 127requirements and/or sacrificing wet-cake purity.

Case E: This case is similar to Case D, except that in accordance withFIG. 5, wet-cake product 119 recovered by basket centrifuge 117 insteadof primary crystallization product slurry 113 from the adiabaticcrystallizer is transferred and mixed with evaporative crystallizationslurry 126 from the non-adiabatic evaporative crystallizer 125 in theevaporative crystallizer centrifuge feed tank 141. In this case, similarsecond wet-cake product 151 purity and quantity is achieved as in CaseD, but at a reduced non-adiabatic evaporative crystallizer overhead 127requirement.

Table 2 below summarizes the input and model calculated values for thesimulated material balance in Cases A through E.

TABLE 2 Water in Aqueous Ratio of Feed Stream wt % Gly Primary 101 to inFraction % of Primary Reactor Oxidation 107 to Product Non-Adiabatic %of Total System 103 Reaction Reaction Slurry 113 Crystallizer Production(lb/100 lb Solution Solution Sent to Feed Overhead 127 as Wet-Cake CaseGI)** 105* 105* Tank 141* (lb/lb GI)** 151** A 337.00 9.1% 0.69 0.0%2.73 31.11% B 337.00 9.1% 0.69 0.0% 2.79 35.57% C 418.00 9.1% 0.57 0.0%3.69 41.90% D 314.42 9.1% 0.76 25.0% 2.48 42.55% E 293.55 9.1% 0.7525.0% 2.28 42.53% % of Non- Ratio of Adiabatic Centrate 134 CrystallizerSent to Wet- Wet-Cake Wet-Cake Slurry 126 Cake 121 to 134 % of Wet-Cake151 Gly 121 Gly to Solid Recycled to 151 from Assay Assay BowlNon-Adiabatic Adiabatic (wt % dry (wt % dry Centrifuge Crystallizer CaseCrystallizer** basis)** basis)** 133* 125* A 0.00% 95.00% 97.30% 20% 0.7B 0.00% 93.91% 98.04% 10% 0.75 C 0.00% 94.99% 97.90% 20% 0.18 D 28.22%95.94% 97.17% 20% 0.5 E 28.25% 95.99% 97.15% 20% 0.5 *User Inputs**Model Calculations

The present invention is not limited to the above embodiments and can bevariously modified. The above description of preferred embodiments isintended only to acquaint others skilled in the art with the invention,its principles and its practical application so that others skilled inthe art may adapt and apply the invention in its numerous forms, as maybe best suited to the requirements of a particular use.

With reference to the use of the word(s) “comprise” or “comprises” or“comprising” in this entire specification (including the claims below),it is noted that unless the context requires otherwise, those words areused on the basis and clear understanding that they are to beinterpreted inclusively, rather than exclusively, and that it isintended each of those words to be so interpreted in construing thisentire specification.

1. A process for recovering an N-(phosphonomethyl)glycine product from aslurry comprising precipitated N-(phosphonomethyl)glycine productcrystals and a mother liquor, the process comprising: dividing theslurry into plural fractions comprising a first slurry fraction and asecond slurry fraction; separating precipitatedN-(phosphonomethyl)glycine product crystals from said first fraction,thereby producing a first wet-cake product; and separating precipitatedN-(phosphonomethyl)glycine product crystals from said second fraction,thereby producing a second wet-cake product, the ratio of solids contentof the second wet-cake product to the solids content of the firstwet-cake product, as measured by weight percent of solids in said firstand second wet-cake products, being at least about 1.1.
 2. A process asset forth in claim 1, wherein the ratio of solids content of the secondwet-cake product to the solids content of the first wet-cake product, asmeasured by weight percent of solids in said first and second wet-cakeproducts, is at least about 1.2.
 3. A process as set forth in claim 2,wherein the ratio of solids content of the second wet-cake product tothe solids content of the first wet-cake product, as measured by weightpercent of solids in said first and second wet-cake products, is atleast about 1.25.
 4. A process as set forth in claim 1, whereinprecipitated N-(phosphonomethyl)glycine crystals are separated from saidfirst and second slurry fractions in separate liquid/solids separationdevices.
 5. A process as set forth in claim 4, wherein precipitatedN-(phosphonomethyl)glycine crystals are separated from said first andsecond slurry fractions in parallel in separate centrifuges.
 6. Aprocess as set forth in claim 5, wherein precipitatedN-(phosphonomethyl)glycine crystals are separated from said first slurryfraction in a solid bowl centrifuge and precipitatedN-(phosphonomethyl)glycine crystals are separated from said secondslurry fraction in a basket centrifuge.
 7. A process as set forth inclaim 6, wherein precipitated N-(phosphonomethyl)glycine crystals areseparated from said second slurry fraction in multiple basketcentrifuges.
 8. A process as set forth in claim 1, wherein the secondwet-cake product has a solids content of at least about 85% by weightsolids.
 9. A process as set forth in claim 8, wherein the secondwet-cake product has a solids content of from about 90% by weight solidsto about 99% by weight solids.
 10. A process as set forth in claim 9,wherein the second wet-cake product has a solids content of from about95% by weight solids to about 99% by weight solids.
 11. A process as setforth in claim 1, wherein the first wet-cake product has a solidscontent of less than about 85% by weight solids.
 12. A process as setforth in claim 11, wherein the first wet-cake product has a solidscontent of less than about 75% by weight solids.
 13. A process as setforth in claim 1, wherein the first wet-cake product has a solidscontent of from about 70% by weight solids to about 85% by weightsolids.
 14. A process as set forth in claim 1, wherein said firstfraction divided from said slurry constitutes from about 20% to about100% of the slurry.
 15. A process as set forth in claim 14, wherein thefirst fraction divided from said slurry constitutes from about 40% toabout 60% of the slurry.
 16. A process as set forth in claim 15, whereinthe first fraction divided from said slurry constitutes about 50% of theslurry.
 17. A process for recovering an N-(phosphonomethyl)glycineproduct from a slurry comprising precipitated N-(phosphonomethyl)glycineproduct crystals and a mother liquor, the process comprising: dividingthe slurry into plural fractions comprising a first slurry fraction anda second slurry fraction; introducing said first slurry fraction into afirst liquid/solids separation device; separating precipitatedN-(phosphonomethyl)glycine product crystals from said first slurryfraction, thereby producing a first wet-cake product; introducing saidsecond slurry fraction into a second liquid/solids separation device inparallel with said first liquid/solids separation device; and separatingprecipitated N-(phosphonomethyl)glycine product crystals from saidsecond slurry fraction, thereby producing a second wet-cake product, thesecond wet-cake product having a greater solids content than the firstwet-cake product as measured by weight percent of solids in saidwet-cake products.
 18. A process as set forth in claim 17, wherein theratio of solids content of the second wet-cake product to the solidscontent of the first wet-cake product, as measured by weight percent ofsolids in said first and second wet-cake products, is at least about1.1.
 19. A process as set forth in claim 18, wherein the ratio of solidscontent of the second wet-cake product to the solids content of thefirst wet-cake product, as measured by weight percent of solids in saidfirst and second wet-cake products, is at least about 1.2.
 20. A processas set forth in claim 19, wherein the ratio of solids content of thesecond wet-cake product to the solids content of the first wet-cakeproduct, as measured by weight percent of solids in said first andsecond wet-cake products, is at least about 1.25.
 21. A process as setforth in claim 17, wherein precipitated N-(phosphonomethyl)glycineproduct crystals are separated from said first and second slurryfractions in parallel in separate centrifuges.
 22. A process as setforth in claim 21, wherein precipitated N-(phosphonomethyl)glycineproduct crystals are separated from said first slurry fraction in asolid bowl centrifuge and precipitated N-(phosphonomethyl)glycineproduct crystals are separated from said second slurry fraction in abasket centrifuge.
 23. A process as set forth in claim 22, whereinprecipitated N-(phosphonomethyl)glycine product crystals are separatedfrom said second slurry fraction in multiple basket centrifuges.
 24. Aprocess as set forth in claim 17, wherein the second wet-cake producthas a solids content of at least about 85% by weight solids.
 25. Aprocess as set forth in claim 24, wherein the second wet-cake producthas a solids content of from about 90% by weight solids to about 99% byweight solids.
 26. A process as set forth in claim 25, wherein thesecond wet-cake product has a solids content of from about 95% by weightsolids to about 99% by weight solids.
 27. A process as set forth inclaim 17, wherein the first wet-cake product has a solids content ofless than about 85% by weight solids.
 28. A process as set forth inclaim 27, wherein the first wet-cake product has a solids content ofless than about 75% by weight solids.
 29. A process as set forth inclaim 17, wherein the first wet-cake product has a solids content offrom about 70% by weight solids to about 85% by weight solids.